Process for degrading nitrile rubbers in the presence of catalysts having an increased activity

ABSTRACT

A process of degrading nitrile rubbers comprises subjecting them to a metathesis reaction in the presence of specific catalysts with increased activity.

This application is a continuation of U.S. patent application Ser. No.11/707,423 filed Feb. 16, 2007, pending, incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a process for degrading nitrile rubbersby subjecting such nitrile rubbers to a metathesis reaction in thepresence of specific catalysts.

BACKGROUND OF THE INVENTION

The term nitrile rubber, also referred to as “NBR” for short, refers torubbers which are copolymers or terpolymers of at least oneα,β-unsaturated nitrile, at least one conjugated diene and, if desired,one or more further copolymerizable monomers.

Hydrogenated nitrile rubber, also referred to as “HNBR” for short, isproduced by hydrogenation of nitrile rubber. Accordingly, the C═C doublebonds of the copolymerized diene units have been completely or partlyhydrogenated in HNBR. The degree of hydrogenation of the copolymerizeddiene units is usually in the range from 50 to 100%.

Hydrogenated nitrile rubber is a specialty rubber which has very goodheat resistance, an excellent resistance to ozone and chemicals and alsoan excellent oil resistance.

The abovementioned physical and chemical properties of HNBR areassociated with very good mechanical properties, in particular a highabrasion resistance. For this reason, HNBR has found wide use in avariety of applications. HNBR is used, for example, for seals, hoses,belts and clamping elements in the automobile sector, also for stators,oil well seals and valve seals in the field of oil extraction and alsofor numerous parts in the aircraft industry, the electronics industry,mechanical engineering and shipbuilding.

Commercially available HNBR grades usually have a Mooney viscosity (ML1+4 at 100° C.) in the range from 55 to 105, which corresponds to aweight average molecular weight M_(w) (method of determination: gelpermeation chromatography (GPC) against polystyrene equivalents) in therange from about 200 000 to 500 000. The polydispersity index PDI(PDI=M_(w)/M_(n), where M_(w) is the weight average molecular weight andM_(n) is the number average molecular weight), which gives informationabout the width of the molecular weight distribution, measured here isfrequently 3 or above. The residual double bond content is usually inthe range from 1 to 18% (determined by IR spectroscopy).

The processability of HNBR is subject to severe restrictions as a resultof the relatively high Mooney viscosity. For many applications, it wouldbe desirable to have an HNBR grade which has a lower molecular weightand thus a lower Mooney viscosity. This would decisively improve theprocessability.

Numerous attempts have been made in the past to shorten the chain lengthof HNBR by degradation. For example, the molecular weight can bedecreased by thermomechanical treatment (mastication), e.g. on a rollmill or in a screw apparatus (EP-A-0 419 952). However, thisthermomechanical degradation has the disadvantage that functional groupssuch as hydroxyl, keto, carboxyl and ester groups, are incorporated intothe molecule as a result of partial oxidation and, in addition, themicrostructure of the polymer is substantially altered.

The preparation of HNBR having low molar masses corresponding to aMooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a numberaverage molecular weight of about M_(n)<200 000 g/mol was for a longtime not possible by means of established production processes since,firstly, a steep increase in the Mooney viscosity occurs in thehydrogenation of NBR and, secondly, the molar mass of the NBR feedstockused for the hydrogenation cannot be reduced at will since otherwise thework-up can no longer be carried out in the industrial plants availablebecause the product is too sticky. The lowest Mooney viscosity of an NBRfeedstock which can be processed without difficulties in an establishedindustrial plant is about 30 Mooney units (ML 1+4 at 100° C.). TheMooney viscosity of the hydrogenated nitrile rubber obtained using suchan NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.).

In the more recent prior art, this problem is solved by reducing themolecular weight of the nitrile rubber prior to hydrogenation bydegradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30Mooney units or a number average molecular weight of M_(n)<70 000 g/mol.The decrease in the molecular weight is achieved here by metathesis inwhich low molecular weight 1-olefins are usually added. The metathesisreaction is advantageously carried out in the same solvent as thehydrogenation reaction (in situ) so that the degraded NBR feedstock doesnot have to be isolated from the solvent after the degradation reactionis complete before it is subjected to the subsequent hydrogenation.Metathesis catalysts which have a tolerance towards polar groups, inparticular towards nitrile groups, are used for catalysing themetathetic degradation reaction.

WO-A-02/100905 and WO-A-02/100941 describe a process which comprisesdegradation of nitrile rubber starting polymers by olefin metathesis andsubsequent hydrogenation. Here, a nitrile rubber is reacted in a firststep in the presence of a coolefin and a specific catalyst based onosmium, ruthenium, molybdenum or tungsten complexes and hydrogenated ina second step. Hydrogenated nitrile rubbers having a weight averagemolecular weight (M_(w)) in the range from 30 000 to 250 000, a Mooneyviscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and apolydispersity index PDI of less than 2.5 can be obtained by this routeaccording to WO-A-02/100941.

Metathesis catalysts are known, inter alia, from WO-A-96/04289 andWO-A-97/06185. They have the following in-principle structure:

where M is osmium or ruthenium, R and R₁ are organic radicals having awide range of structural variation, X and X₁ are anionic ligands and Land L₁ are uncharged electron donors. The customary term “anionicligands” is used in the literature regarding such metathesis catalyststo describe ligands which are always negatively charged with a closedelectron shell when regarded separately from the metal centre.

Such catalysts are suitable for ring-closing metatheses (RCM),cross-metatheses (CM) and ring-opening metatheses (ROMP). However, thecatalysts mentioned are not necessarily suitable for carrying out thedegradation of nitrile rubber.

The metathesis of nitrile rubber can be successfully carried out usingsome catalysts from the group of “Grubbs (I) catalysts”. A suitablecatalyst is, for example, a ruthenium catalyst having particularsubstitution patterns, e.g. the catalystbis(tricyclohexylphosphine)benzylideneruthenium dichloride shown below.

After metathesis and hydrogenation, the nitrile rubbers have a lowermolecular weight and also a narrower molecular weight distribution thanthe hydrogenated nitrile rubbers which have hitherto been able to beprepared according to the prior art.

However, the amounts of Grubbs (I) catalyst employed for carrying outthe metathesis are large. In the experiments in WO-A-03/002613, theyare, for example, 307 ppm and 61 ppm of Ru based on the nitrile rubberused. The reaction times necessary are also long and the molecularweights after the degradation are still relatively high (see Example 3of WO-A-03/002613, in which M_(w)=180 000 g/mol and M_(n)=71 000 g/mol).

US 2004/0127647 A1 describes blends based on low molecular weight HNBRrubbers having a bimodal or multimodal molecular weight distribution andalso vulcanisates of these rubbers. To carry out the metathesis, 0.5 phrof Grubbs I catalyst, corresponding to 614 ppm of ruthenium based on thenitrile rubber used, is used according to the examples.

Furthermore, WO-A-00/71554 discloses a group of catalysts which areknown in the technical field as “Grubbs (II) catalysts”.

If such a “Grubbs (II) catalyst”, e.g.1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidenylidene)(tricyclohexylphosphine)ruthenium(phenylmethylene)dichloride, is used for the NBR metathesis, this also succeeds withoutuse of a coolefin (US-A-2004/0132891). After the subsequenthydrogenation, which is preferably carried out in situ, the hydrogenatednitrile rubber has lower molecular weights and a narrower molecularweight distribution (PDI) than when using catalysts of the Grubbs (I)type. In terms of the molecular weight and the molecular weightdistribution, the metathetic degradation thus proceeds more efficientlywhen using catalysts of the Grubbs II type than when using catalysts ofthe Grubbs I type. However, the amounts of ruthenium necessary for thisefficient metathetic degradation are still relatively high. Longreaction times are also still required for carrying out the metathesisusing the Grubbs II catalyst.

In all the abovementioned processes for the metathetic degradation ofnitrile rubber, relatively large amounts of catalyst have to be used andlong reaction times are required in order to produce the desired lowmolecular weight nitrile rubbers.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a catalyst whichmakes metathetic degradation of nitrile rubber without gelling possible,at the same time at a higher activity than the metathesis catalystsavailable at present and thus makes possible an increase in the reactionrate and the setting of lower molecular weights of the degraded nitrilerubber at a comparable noble metal content.

This object has surprisingly been able to be achieved by a process fordegrading nitrile rubbers by subjecting them to a metathesis reaction inthe presence of specific catalysts of the general formula (I) shownbelow.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides for a process for degrading nitrile rubberscomprising subjecting such nitrile rubbers to a metathesis reaction inthe presence of a catalyst of the general formula (I)

where

-   M is ruthenium or osmium,-   Y is oxygen (O), sulphur (S), an N—R¹ radical or a P—R¹ radical,    where R¹ is as defined below,-   X¹ and X² are identical or different ligands,-   R¹ is an alkyl, cycloalkyl, alkenyl, alkynyl, aryl, alkoxy,    alkenyloxy, alkynyloxy, aryloxy, alkoxycarbonyl, alkylamino,    alkylthio, arylthio, alkylsulphonyl or alkylsulphinyl radical, each    of which may optionally be substituted by one or more alkyl,    halogen, alkoxy, aryl or heteroaryl radicals,-   R², R³, R⁴ and R⁵ are identical or different and are each hydrogen,    organic or inorganic radicals,-   R⁶ is hydrogen or an alkyl, alkenyl, alkynyl or aryl radical and-   L is a ligand.

The catalysts of the general formula (I) are known in principle.Representatives of this class of compounds are the catalysts describedby Hoveyda et al. in US 2002/0107138 A1 and Angew Chem. Int. Ed. 2003,42, 4592, and the catalysts described by Grela in WO-A-2004/035596, Eur.J. Org. Chem. 2003, 963-966 and Angew. Chem. Int. Ed. 2002, 41, 4038 andin J. Org. Chem. 2004, 69, 6894-96 and Chem. Eur. J. 2004, 10, 777-784.The catalysts are commercially available or can be prepared as describedin the references cited.

It is surprisingly possible to carry out the metathetic degradation ofnitrile rubber without gel formation using the catalysts having thestructural features of the general formula (I), with such catalystsadditionally displaying a higher activity than Grubbs II catalysts.

The term “substituted” used for the purposes of the present patentapplication means that a hydrogen atom on an indicated radical or atomhas been replaced by one of the groups indicated in each case, with theproviso that the valency of the atom indicated is not exceeded and thesubstitution leads to a stable compound.

For the purposes of the present patent application and invention, allthe definitions of radicals, parameters or explanations given above orbelow in general terms or in preferred ranges can be combined with oneanother in any way, i.e. including combinations of the respective rangesand preferred ranges.

In the catalysts of the general formula (I), L is a ligand, usually aligand having an electron donor function. L can be a P(R⁷)₃ radical,where the radicals R⁷ are each, independently of one another,C₁-C₆-alkyl, C₃-C₈-cycloalkyl or aryl or a substituted or unsubstitutedimidazolidine radical (“Im”).

C₁-C₆-Alkyl is, for example, methyl, ethyl, n-propyl, isopropyl,n-butyl, sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, neopentyl, 1-ethylpropyl or n-hexyl.

C₃-C₈-Cycloalkyl encompasses cyclopropyl, cyclobutyl, cyclopentyl,cyclohexyl, cycloheptyl and cyclooctyl.

Aryl encompasses an aromatic radical having from 6 to 24 skeletal carbonatoms. Preferred monocyclic, bicyclic or tricyclic carbocyclic aromaticradicals having from 6 to 10 skeletal carbon atoms are, for example,phenyl, biphenyl, naphthyl, phenanthrenyl and anthracenyl.

The imidazolidine radical (Im) usually has a structure of the generalformula (IIa) or (IIb),

where

-   R⁸, R⁹, R¹⁰, R¹¹ are identical or different and are each hydrogen,    straight-chain or branched C₁-C₃₀-alkyl, preferably C₁-C₂₀-alkyl,    C₃-C₂₀-cycloalkyl, preferably C₃-C₁₀-cycloalkyl, C₂-C₂₀-alkenyl,    preferably C₂-C₁₀-alkenyl, C₂-C₂₀-alkynyl, preferably    C₂-C₁₀-alkynyl, C₆-C₂₄-aryl, preferably C₆-C₁₄-aryl,    C₁-C₂₀-carboxylate, preferably C₁-C₁₀-carboxylate, C₁-C₂₀-alkoxy,    preferably C₁-C₁₀-alkoxy, C₂-C₂₀-alkenyloxy, preferably    C₂-C₁₀-alkenyloxy, C₂-C₂₀-alkynyloxy, preferably C₂-C₁₀-alkynyloxy,    C₆-C₂₀-aryloxy, preferably C₆-C₁₄-aryloxy, C₂-C₂₀-alkoxycarbonyl,    preferably C₂-C₁₀-alkoxycarbonyl, C₁-C₂₀-alkylthio, preferably    C₁-C₁₀-alkylthio, C₆-C₂₀-arylthio, preferably C₆-C₁₄-arylthio,    C₁-C₂₀-alkylsulphonyl, preferably C₁-C₁₀-alkylsulphonyl,    C₁-C₂₀-alkylsulphonate, preferably C₁-C₁₀-alkylsulphonate,    C₆-C₂₀-arylsulphonate, preferably C₆-C₁₄-arylsulphonate, or    C₁-C₂₀-alkylsulphinyl, preferably C₁-C₁₀-alkylsulphinyl.

One or more of the radicals R⁸, R⁹, R¹⁰, R¹¹ may, independently of oneanother, optionally be substituted by one or more substituents,preferably straight-chain or branched C₁-C₁₀-alkyl, C₃-C₈-cycloalkylC₁-C₁₀-alkoxy or C₆-C₂₄-aryl, where these abovementioned substituentsmay in turn be substituted by one or more radicals, preferably selectedfrom the group consisting of halogen, in particular chlorine or bromine,C₁-C₅-alkyl, C₁-C₅-alkoxy and phenyl.

In a preferred embodiment of the novel catalysts of the general formula(I), R⁸ and R⁹ are each, independently of one another, hydrogen,C₆-C₂₄-aryl, particularly preferably phenyl, straight-chain or branchedC₁-C₁₀-alkyl, particularly preferably propyl or butyl, or together form,with inclusion of the carbon atoms to which they are bound, a cycloalkylor aryl radical, where all the abovementioned radicals may in turn besubstituted by one or more further radicals selected from the groupconsisting of straight-chain or branched C₁-C₁₀-alkyl, C₁-C₁₀-alkoxy,C₆-C₂₄-aryl and functional groups selected from the group consisting ofhydroxy, thiol, thioether, ketone, aldehyde, ester, ether, amine, imine,amide, nitro, carboxyl, disulphide, carbonate, isocyanate, carbodiimide,carboalkoxy, carbamate and halogen.

In a preferred embodiment of the novel catalysts, the radicals R¹⁰ andR¹¹ are identical or different and are each straight-chain or branchedC₁-C₁₀-alkyl, particularly preferably i-propyl or neopentyl,C₃-C₁₀-cycloalkyl, preferably adamantyl, C₆-C₂₄-aryl, particularlypreferably phenyl, C₁-C₁₀-alkylsulphonate, particularly preferablymethanesulphonate, C₆-C₁₀-arylsulphonate, particularly preferablyp-toluenesulphonate.

These radicals R¹⁰ and R¹¹ which are mentioned above as being preferredmay optionally be substituted by one or more further radicals selectedfrom the group consisting of straight-chain or branched C₁-C₅-alkyl, inparticular methyl, C₁-C₅-alkoxy, aryl and functional groups selectedfrom the group consisting of hydroxy, thiol, thioether, ketone,aldehyde, ester, ether, amine, imine, amide, nitro, carboxyl,disulphide, carbonate, isocyanate, carbodiimide, carboalkoxy, carbamateand halogen.

In particular, the radicals R¹⁰ and R¹¹ are identical or different andare each i-propyl, neopentyl, adamantyl or mesityl.

Particularly preferred imidazolidine radicals (Im) have the structures(IIIa-f), where Mes is in each case a 2,4,6-trimethylphenyl radical.

In the catalysts of the general formula (I), X¹ and X² are identical ordifferent and can be, for example, hydrogen, halogen, pseudohalogen,straight-chain or branched C₁-C₃₀-alkyl, C₆-C₂₄-aryl, C₁-C₂₀-alkoxy,C₆-C₂₄-aryloxy, C₃-C₂₀-alkyldiketonate, C₆-C₂₄-aryldiketonate,C₁-C₂₀-carboxylate, C₁-C₂₀-alkylsulphonate, C₆-C₂₄-arylsulphonate,C₁-C₂₀-alkylthiol, C₆-C₂₄-arylthiol, C₁-C₂₀-alkylsulphonyl orC₁-C₂₀-alkylsulphinyl.

The abovementioned radicals X¹ and X² can also be substituted by one ormore further radicals, for example by halogen, preferably fluorine,C₁-C₁₀-alkyl, C₁-C₁₀-alkoxy or C₆-C₂₄-aryl radicals, where the latterradicals may optionally also in turn be substituted by one or moresubstituents selected from the group consisting of halogen, preferablyfluorine, C₁-C₅-alkyl, C₁-C₅-alkoxy and phenyl.

In a preferred embodiment, X¹ and X² are identical or different and areeach halogen, in particular fluorine, chlorine or bromine, benzoate,C₁-C₅-carboxylate, C₁-C₅-alkyl, phenoxy, C₁-C₅-alkoxy, C₁-C₅-alkylthiol,C₆-C₂₄-arylthiol, C₆-C₂₄-aryl or C₁-C₅-alkylsulphonate.

In a particularly preferred embodiment, X¹ and X² are identical and areeach halogen, in particular chlorine, CF₃COO, CH₃COO, CFH₂COO, (CH₃)₃CO,(CF₃)₂(CH₃)CO, (CF₃)(CH₃)₂CO, PhO (phenoxy), MeO (methoxy), EtO(ethoxy), tosylate (p-CH₃—C₆H₄—SO₃), mesylate (2,4,6-trimethylphenyl) orCF₃SO₃ (trifluoromethanesulphonate).

In the general formula (I), the radical R¹ is an alkyl, cycloalkyl,alkenyl, alkynyl, aryl, alkoxy, alkenyloxy, alkynyloxy, aryloxy,alkoxycarbonyl, alkylamino, alkylthio, arylthio, alkylsulphonylalkylsulphinyl radical, each of which may optionally be substituted byone or more alkyl, halogen, alkoxy, aryl or heteroaryl radicals.

The radical R¹ is usually a C₁-C₃₀-alkyl, C₃-C₂₀-cycloalkyl,C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl, C₁-C₂₀-alkoxy,C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₄-aryloxy,C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylamino, C₁-C₂₀-alkylthio,C₆-C₂₄-arylthio, C₁-C₂₀-alkylsulphonyl or C₁-C₂₀-alkylsulphinyl radical,each of which may optionally be substituted by one or more alkyl,halogen, alkoxy, aryl or heteroaryl radicals.

R¹ is preferably a C₃-C₂₀-cycloalkyl radical, a C₆-C₂₄-aryl radical or astraight-chain or branched C₁-C₃₀-alkyl radical, with the latteroptionally being able to be interrupted by one or more double or triplebonds or one or more heteroatoms, preferably oxygen or nitrogen. R¹ isparticularly preferably a straight-chain or branched C₁-C₁₂-alkylradical.

The C₃-C₂₀-cycloalkyl radical encompasses, for example, cyclopropyl,cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl.

The C₁-C₁₂-alkyl radical can be, for example, methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl,2-methylbutyl, 3-methylbutyl, neopentyl, 1-ethylpropyl, n-hexyl,n-heptyl, n-octyl, n-decyl or n-dodecyl. In particular, R¹ is methyl orisopropyl.

The C₆-C₂₄-aryl radical is an aromatic radical having from 6 to 24skeletal carbon atoms. As particularly preferred monocyclic, bicyclic ortricyclic carbocyclic aromatic radicals having from 6 to 10 skeletalcarbon atoms, mention may be made by way of example of phenyl, biphenyl,naphthyl, phenanthrenyl or anthracenyl.

In the general formula (I), the radicals R², R³, R⁴ and R⁵ are identicalor different and can be hydrogen, organic or inorganic radicals.

In a preferred embodiment, R², R³, R⁴, R⁵ are identical or different andare each hydrogen, halogen, nitro, CF₃, alkyl, cycloalkyl, alkenyl,alkynyl, aryl, alkoxy, alkenyloxy, alkynyloxy, aryloxy, alkoxycarbonyl,alkylamino, alkylthio, arylthio, alkylsulphonyl or alkylsulphinyl, eachof which may optionally be substituted by one or more alkyl, alkoxy,halogen, aryl or heteroaryl radicals.

R², R³, R⁴, R⁵ are usually identical or different and are each hydrogen,halogen, preferably chlorine or bromine, nitro, CF₃, C₁-C₃₀-alkyl,C₃-C₂₀-cycloalkyl, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl,C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₄-aryloxy,C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylamino, C₁-C₂₀-alkylthio,C₆-C₂₄-arylthio, C₁-C₂₀-alkylsulphonyl or C₁-C₂₀-alkylsulphinyl, each ofwhich may optionally be substituted by one or more C₁-C₃₀-alkyl,C₁-C₂₀-alkoxy, halogen, C₆-C₂₄-aryl or heteroaryl radicals.

In a particularly useful embodiment, R², R³, R⁴, R⁵ are identical ordifferent and are each nitro, a straight-chain or branched C₁-C₃₀-alkylor C₆-C₂₀-cycloalkyl radical, a straight-chain or branched C₁-C₂₀-alkoxyradical or a C₆-C₂₄-aryl radical, preferably phenyl or naphthyl. TheC₁-C₃₀-alkyl radicals and C₁-C₂₀-alkoxy radicals may optionally beinterrupted by one or more double or triple bonds or one or moreheteroatoms, preferably oxygen or nitrogen.

Furthermore, two or more of the radicals R², R³, R⁴ or R⁵ can be bridgedvia aliphatic or aromatic structures. For example, R³ and R⁴ can, withinclusion of the carbon atoms to which they are bound in the phenyl ringof the formula (I), form a fused-on phenyl ring so that overall anaphthyl structure results.

In the general formula (I), R⁶ is hydrogen or an alkyl, alkenyl, alkynylor aryl radical. R⁶ is preferably hydrogen or a C₁-C₃₀-alkyl radical, aC₂-C₂₀-alkenyl radical, a C₂-C₂₀-alkynyl radical or a C₆-C₂₄-arylradical. R⁶ is particularly preferably hydrogen.

Particularly suitable catalysts for the process according to theinvention are catalysts of the general formula (IV)

where

-   M, L, X¹, X², R¹, R², R³, R⁴ and R⁵ have the meanings given for the    general formula (I).

These catalysts are known in principle, for example from US 2002/0107138A1 (Hoveyda et al.), and can be obtained by preparative methodsindicated there.

Particular preference is given to catalysts of the general formula (IV)in which

-   M is ruthenium,-   X¹ and X² are both halogen, in particular, both chlorine,-   R¹ is a straight-chain or branched C₁-C₁₂-alkyl radical,-   R², R³, R⁴, R⁵ have the meanings given for the general formula (I)    and-   L has the meanings given for the general formula (I).

Very particular preference is given to catalysts of the general formula(IV) in which

-   M is ruthenium,-   X¹ and X² are both chlorine,-   R¹ is an isopropyl radical,-   R², R³, R⁴, R⁵ are all hydrogen and-   L is a substituted or unsubstituted imidazolidine radical of the    formula (IIa) or (IIb),

where

-   R⁸, R⁹, R¹⁰, R¹¹ are identical or different and are each hydrogen,    straight-chain or branched C₁-C₃₀-alkyl, C₃-C₂₀-cycloalkyl,    C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl, C₁-C₂₀-carboxylate,    C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₄-aryloxy,    C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylthio, C₆-C₂₄-arylthio,    C₁-C₂₀-alkylsulphonyl, C₁-C₂₀-alkylsulphonate, C₆-C₂₄-arylsulphonate    or C₁-C₂₀-alkylsulphinyl.

A very particularly preferred catalyst which comes under the generalstructural formula (IV) is that of the formula (V)

which is also referred to as “Hoveyda catalyst” in the literature.

Further suitable catalysts which come under the general structuralformula (IV) are those of the formulae (VI), (VII), (VIII), (IX), (X),(XI), (XII) and (IXX), where Mes is in each case a 2,4,6-trimethylphenylradical.

Further catalysts which are particularly suitable for the processaccording to the invention are catalysts of the general formula (XIII)

where

-   M, L, X¹, X², R¹ and R⁶ have the meanings given for the general    formula (I),-   the radicals R¹² are identical or different and have the meanings    given for the radicals R², R³, R⁴ and R⁵, with the exception of    hydrogen, and-   n is 0, 1, 2 or 3.

These catalysts are known in principle, for example fromWO-A-2004/035596 (Grela), and can be obtained by the preparative methodsindicated there.

Particular preference is given to catalysts of the general formula (XII)in which

-   M is ruthenium,-   X¹ and X² are both halogen, in particular both chlorine,-   R¹ is a straight-chain or branched C₁-C₁₂-alkyl radical,-   R¹² has the meanings given for the general formula (I),-   n is 0, 1, 2 or 3 and-   L has the meanings given for the general formula (I).

Very particular preference is given to catalysts of the general formula(XII) in which

-   M is ruthenium,-   X¹ and X² are both chlorine,-   R¹ is an isopropyl radical,-   n is 0 and-   L is a substituted or unsubstituted imidazolidine radical of the    formula (IIa) or (IIb),

where

-   R⁸, R⁹, R¹⁰, R¹¹ are identical or different and are each hydrogen,    straight-chain or branched, cyclic or acyclic C₁-C₃₀-alkyl,    C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl, C₁-C₂₀-carboxylate,    C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₀-aryloxy,    C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylthio, C₆-C₄-arylthio,    C₁-C₂₀-alkylsulphonyl, C₁-C₂₀-alkylsulphonate, C₆-C₄-arylsulphonate    or C₁-C₂₀-alkylsulphinyl.

A particularly suitable catalyst which comes under the general formula(XIII) has the structure (XIV)

and is also referred to in the literature as “Grela catalyst”.

A further suitable catalyst which comes under the general formula (XIII)has the structure (XV).

In an alternative embodiment, it is also possible to use dendriticcatalysts of the general formula (XVI),

where D¹, D², D³ and D⁴ each have a structure of the general formula(XVII) which is bound via the methylene group to the silicon of theformula (XVI),

where

-   M, L, X¹, X², R¹, R², R³, R⁵ and R⁶ have the meanings given for the    general formula (I) or can have the meanings given for all the    abovementioned preferred or particularly preferred embodiments.

Such catalysts of the general formula (XVI) are known from US2002/0107138 A1 and can be prepared according to the information giventhere.

In a further alternative embodiment, it is possible to perform theinventive process in the presence of a catalyst of the general formula(XVIII),

where the symbol

represents a support.

The support is preferably a poly(styrene-divinylbenzene)copolymer(PS-DVB).

These catalysts of the formula (XVIII) are known in principle from Chem.Eur. J. 2004 10, 777-784 and can be obtained by the preparative methodsdescribed there.

All the abovementioned catalysts of the formulae (I), (IV)-(XVI),(XVIII) and (IXX) can either be used as such for the NBR metathesis orcan be applied to and immobilized on a solid support. As solid phases orsupports, it is possible to use materials which firstly are inerttowards the reaction mixture of the metathesis and secondly do notimpair the activity of the catalyst. It is possible to use, for example,metals, glass, polymers, ceramic, organic polymer spheres or inorganicsol-gels for immobilizing the catalyst.

The catalysts of all the abovementioned general and specific formulae(I), (IV)-(XVI), (XVIII) and (IXX) are highly suitable for themetathetic degradation of nitrile rubber.

In the process according to the invention, a nitrile rubber is subjectedto a metathesis reaction in the presence of a catalyst of the generalformula (I).

The amount of the catalyst used according to the invention for themetathesis depends on the nature and the catalytic activity of thespecific catalyst. The amount of catalyst used is from 5 to 1000 ppm ofnoble metal, preferably from 5 to 500 ppm, in particular from 5 to 250ppm, based on the nitrile rubber used.

The NBR metathesis can be carried out without a coolefin or in thepresence of a coolefin. This is preferably a straight-chain or branchedC₂-C₁₆-olefin. Suitable coolefins are, for example, ethylene, propylene,isobutene, styrene, 1-hexene and 1-octene. Preference is given to using1-hexene or 1-octene. If the coolefin is liquid (as in the case of, forexample, 1-hexene), the amount of coolefin is preferably in the range0.2-20% by weight based on the nitrile rubber used. If the coolefin is agas, as in the case of, for example, ethylene, the amount of coolefin isselected so that a pressure in the range 1×10⁵ Pa-1×10⁷ Pa, preferably apressure in the range from 5.2×10⁵ Pa to 4×10⁶ Pa, is established in thereaction vessel at room temperature.

The metathesis reaction can be carried out in a suitable solvent whichdoes not deactivate the catalyst used and also does not adversely affectthe reaction in any other way. Preferred solvents include but are notrestricted to dichloromethane, benzene, toluene, methyl ethyl ketone,acetone, tetrahydrofuran, tetrahydropyran, dioxane and cyclohexane. Theparticularly preferred solvent is chlorobenzene. In some cases, when thecoolefin itself can function as solvent, e.g. in the case of 1-hexene,the addition of a further additional solvent can also be omitted.

The concentration of the nitrile rubber used in the reaction mixture ofthe metathesis is not critical, but care naturally has to be taken toensure that the reaction is not adversely affected by an excessivelyhigh viscosity of the reaction mixture and the mixing problemsassociated therewith. The concentration of NBR in the reaction mixtureis preferably in the range from 1 to 20% by weight, particularlypreferably in the range from 5 to 15% by weight, based on the totalreaction mixture.

The metathetic degradation is usually carried out at a temperature inthe range from 10° C. to 150° C., preferably in the range from 20° C. to100° C.

The reaction time depends on a number of factors, for example, on thetype of NBR, the type of catalyst, the catalyst concentration used andthe reaction temperature. The reaction is typically complete withinthree hours under normal conditions. The progress of the metathesis canbe monitored by standard analytical methods, e.g. by GPC measurement orby determination of the viscosity.

As nitrile rubbers (“NBR”), it is possible to use copolymers orterpolymers which comprise repeating units of at least one conjugateddiene, at least one α,β-unsaturated nitrile and, if desired, one or morefurther copolymerizable monomers in the metathesis reaction.

The conjugated diene can be of any nature. Preference is given to using(C₄-C₆) conjugated dienes. Particular preference is given to1,3-butadiene, isoprene, 2,3-dimethylbutadiene, piperylene or mixturesthereof. Very particular preference is given to 1,3-butadiene andisoprene or mixtures thereof. Especial preference is given to1,3-butadiene.

As α,β-unsaturated nitrile, it is possible to use any knownα,β-unsaturated nitrile, preferably a (C₃-C₅) α,β-unsaturated nitrilesuch as acrylonitrile, methacrylonitrile, ethacrylonitrile or mixturesthereof. Particular preference is given to acrylonitrile.

A particularly preferred nitrile rubber is thus a copolymer ofacrylonitrile and 1,3-butadiene.

Apart from the conjugated diene and the α,β-unsaturated nitrile, it ispossible to use one or more further copolymerizable monomers known tothose skilled in the art, e.g. α,β-unsaturated monocarboxylic ordicarboxylic acids, their esters or amides. As α,β-unsaturatedmonocarboxylic or dicarboxylic acids, preference is given to fumaricacid, maleic acid, acrylic acid and methacrylic acid. As esters ofα,β-unsaturated carboxylic acids, preference is given to using theiralkyl esters and alkoxyalkyl esters. Particularly preferred alkyl estersof α,β-unsaturated carboxylic acids are methyl acrylate, ethyl acrylate,butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexylmethacrylate and octyl acrylate. Particularly preferred alkoxyalkylesters of α,β-unsaturated carboxylic acids are methoxyethyl(meth)acrylate, ethoxyethyl (meth)acrylate and methoxyethyl(meth)acrylate. It is also possible to use mixtures of alkyl esters,e.g. those mentioned above, with alkoxyalkyl esters, e.g. in the form ofthose mentioned above.

The proportions of conjugated diene and α,β-unsaturated nitrile in theNBR polymers to be used can vary within wide ranges. The proportion ofor of the sum of the conjugated dienes is usually in the range from 40to 90% by weight, preferably in the range from 55 to 75% by weight,based on the total polymer. The proportion of or of the sum of theα,β-unsaturated nitriles is usually from 10 to 60% by weight, preferablyfrom 25 to 45% by weight, based on the total polymer. The proportions ofthe monomers in each case add up to 100% by weight. The additionalmonomers can be present in amounts of from 0 to 40% by weight,preferably from 0.1 to 40% by weight, particularly preferably from 1 to30% by weight, based on the total polymer. In this case, correspondingproportions of the conjugated diene or dienes and/or of theα,β-unsaturated nitrile or nitriles are replaced by the proportions ofthe additional monomers, with the proportions of all monomers in eachcase adding up to 100% by weight.

The preparation of nitrile rubbers by polymerization of theabovementioned monomers is adequately known to those skilled in the artand is comprehensively described in the polymer literature.

Nitrile rubbers which can be used for the purposes of the invention arealso commercially available, e.g. as products from the product range ofthe trade names Perbunan® and Krynac® from Lanxess Deutschland GmbH.

The nitrile rubbers used for the metathesis have a Mooney viscosity (ML1+4 at 100° C.) in the range from 30 to 70, preferably from 30 to 50.This corresponds to a weight average molecular weight M_(w) in the range200 000-500 000, preferably in the range 200 000-400 000. The nitrilerubbers used also have a polydispersity PDI=M_(w)/M_(n), where M_(w) isthe weight average molecular weight and M_(n) is the number averagemolecular weight, in the range 2.0-6.0 and preferably in the range2.0-4.0.

The determination of the Mooney viscosity is carried out in accordancewith ASTM standard D 1646.

The nitrile rubbers obtained by the metathesis process according to theinvention have a Mooney viscosity (ML 1+4 at 100° C.) in the range 5-30,preferably 5-20. This corresponds to a weight average molecular weightM_(w) in the range 10 000-200 000, preferably in the range 10 000-150000. The nitrile rubbers obtained also have a polydispersityPDI=M_(w)/M_(n), where M_(n) is the number average molecular weight, inthe range 1.5-4.0, preferably in the range 1.7-3.

The metathetic degradation process according to the invention can befollowed by a hydrogenation of the degraded nitrile rubbers obtained.This can be carried out in the manner known to those skilled in the art.

It is possible to carry out the hydrogenation with use of homogeneous orheterogeneous hydrogenation catalysts. It is also possible to carry outthe hydrogenation in situ, i.e. in the same reaction vessel in which themetathetic degradation has previously also been carried out and withoutthe necessity of isolating the degraded nitrile rubber. Thehydrogenation catalyst is simply added to the reaction vessel.

The catalysts used are usually based on rhodium, ruthenium or titanium,but it is also possible to use platinum, iridium, palladium, rhenium,osmium, cobalt or copper either as metal or preferably in the form ofmetal compounds (cf., for example, U.S. Pat. No. 3,700,637, DE-A-25 39132, EP-A-0 134 023, DE-A-35 41 689, DE-A-35 40 918, EP-A-0 298 386,DE-A-35 29 252, DE-A-34 33 392, U.S. Pat. No. 4,464,515 and U.S. Pat.No. 4,503,196).

Suitable catalysts and solvents for a hydrogenation in the homogeneousphase are described below and are also known from DE-A-25 39 132 andEP-A-0 471 250.

The selective hydrogenation can be achieved, for example, in thepresence of a rhodium- or ruthenium-containing catalyst. It is possibleto use, for example, a catalyst of the general formula

(R¹ _(m)B)₁MX_(n),

where M is ruthenium or rhodium, the radicals R¹ are identical ordifferent and are each a C₁-C₈-alkyl group, a C₄-C₈-cycloalkyl group, aC₆-C₁₅-aryl group or a C₇-C₁₅-aralkyl group. B is phosphorus, arsenic,sulphur or a sulphoxide group S═O, X is hydrogen or an anion, preferablyhalogen and particularly preferably chlorine or bromine, 1 is 2, 3 or 4,m is 2 or 3 and n is 1, 2 or 3, preferably 1 or 3. Preferred catalystsare tris(triphenylphosphine)rhodium(I) chloride,tris(triphenylphosphine)rhodium(III) chloride and tris(dimethylsulphoxide)rhodium(I) chloride and alsotetrakis(triphenylphosphine)rhodium hydride of the formula (C₆H₅)₃P)₄RhHand the corresponding compounds in which the triphenylphosphine has beencompletely or partly replaced by tricyclohexylphosphine. The catalystcan be utilized in small amounts. An amount in the range 0.01-1% byweight, preferably in the range 0.03-0.5% by weight and particularlypreferably in the range 0.1-0.3% by weight, based on the weight of thepolymer, is suitable.

It is usually appropriate to use the catalyst together with a cocatalystwhich is a ligand of the formula R¹ _(m)B, where R¹, m and B have themeanings given above for the catalyst. Preferably, m is 3, B isphosphorus and the radicals R¹ can be identical or different. Preferenceis given to cocatalysts having trialkyl, tricycloalkyl, triaryl,triaralkyl, diaryl-monoalkyl, diaryl-monocycloalkyl, dialkyl-monoaryl,dialkyl-monocycloalkyl, dicycloalkyl-monoaryl or dicycloalkyl-monoarylradicals.

Examples of cocatalysts may be found in, for example, U.S. Pat. No.4,631,315. A preferred cocatalyst is triphenylphosphine. The cocatalystis preferably used in amounts in the range 0.3-5% by weight, preferablyin the range 0.5-4% by weight, based on the weight of the nitrile rubberto be hydrogenated. Furthermore, the weight ratio of therhodium-containing catalyst to the cocatalyst is preferably in the rangefrom 1:3 to 1:55, more preferably in the range from 1:5 to 1:45. Basedon 100 parts by weight of the nitrile rubber to be hydrogenated, it isappropriate to use from 0.1 to 33 parts by weight of the cocatalyst,preferably from 0.5 to 20 parts by weight and very particularlypreferably from 1 to 5 parts by weight, in particular more than 2 butless than 5 parts by weight, of cocatalyst per 100 parts by weight ofthe nitrile rubber to be hydrogenated.

The practical implementation of this hydrogenation is adequately knownto those skilled in the art from U.S. Pat. No. 6,683,136. It is usuallycarried out by treating the nitrile rubber to be hydrogenated in asolvent such as toluene or monochlorobenzene with hydrogen at atemperature in the range from 100 to 150° C. and a pressure in the rangefrom 50 to 150 bar for from 2 to 10 hours.

For the purposes of the present invention, hydrogenation is a reactionof the double bonds present in the starting nitrile rubber to an extentof at least 50%, preferably 70-100%, particularly preferably 80-100%.

When heterogeneous catalysts are used, these are usually supportedcatalysts based on palladium which are, for example, supported oncarbon, silica, calcium carbonate or barium sulphate.

After conclusion of the hydrogenation, a hydrogenated nitrile rubberhaving a Mooney viscosity (ML 1+4 at 100° C.), measured in accordancewith ASTM standard D 1646, in the range from 10 to 50, preferably from10 to 30, is obtained. This corresponds to a weight average molecularweight M_(w) in the range 2000-400 000 g/mol, preferably in the range 20000-200 000. The hydrogenated nitrile rubbers obtained also have apolydispersity PDI=M_(w)/M_(n), where M_(w) is the weight averagemolecular weight and M_(n) is the number average molecular weight, inthe range 1-5 and preferably in the range 1.5-3.

EXAMPLES Metathetic Degradation of Nitrile Rubber in the Presence ofVarious Ru Catalysts

In the following examples, it is shown that, in each case at the sameamount of ruthenium, the metathesis activity of the catalysts of thegeneral structural formula (I) is higher than when the Grubbs IIcatalyst is used.

The following catalysts were used:

“Hoveyda Catalyst” (According to the Invention):

The Hoveyda catalyst was procured from Aldrich under the product number569755.

“Grela Catalyst” (According to the Invention):

The Grela catalyst was prepared by the method published in J. Org. Chem.2004, 69, 6894-6896.

Grubbs II Catalyst (Comparison):

The Grubbs II catalyst was procured from Materia (Pasadena/Calif.).

The degradation reactions described below were carried out using thenitrile rubber Perbunan® NT 3435 from Lanxess Deutschland GmbH. Thisnitrile rubber had the following characteristic properties:

Acrylonitrile content: 35% by weightMooney viscosity (ML 1+4 @100° C.): 34 Mooney unitsResidual moisture content: 1.8% by weightM_(w): 240 000 g/molM_(n): 100 000 g/mol

PDI (M_(w)/M_(n)): 2.4

In the text that follows, this nitrile rubber is referred to as NBR forshort.

General Description of the Metathesis Reactions Carried Out

The metathetic degradation was in each case carried out using 293.3 g ofchlorobenzene (hereinafter referred to as “MCB”/from Aldrich) which hadbeen distilled and made inert by passing argon through it at roomtemperature before use. 40 g of NBR were dissolved therein at roomtemperature over a period of 10 hours. 0.8 g (2 phr) of 1-hexene was ineach case added to the NBR-containing solution and the mixture wasstirred for 30 minutes to homogenize it.

The metathesis reactions were carried out using the amounts of startingmaterials indicated in the following tables at room temperature.

The Ru-containing catalysts were in each case dissolved in 20 g of MCBat room temperature under argon. The addition of the catalyst solutionsto the NBR solutions in MCB was carried out immediately after thepreparation of the catalyst solutions. After the reaction timesindicated below in the tables, about 5 ml were in each case taken fromthe reaction solutions and immediately admixed with about 0.2 ml ofethyl vinyl ether to stop the reaction and subsequently diluted with 5ml of DMAc (N,N-dimethylacetamide from Aldrich). 2 ml of the solutionswere in each case placed in a GPC bottle and diluted with DMAc to 3 ml.Before carrying out the GPC analysis, the solutions were in each casefiltered by means of a 0.2 μm syringe filter made of Teflon (ChromafilPTFE 0.2 μm; from Machery-Nagel). The GPC analysis was subsequentlycarried out using a Waters instrument (Mod. 510). The analysis wascarried out using a combination of 4 columns from PolymerLaboratories: 1) PLgel 5 μm Mixed-C, 300×7.5 mm, 2) PLgel 5 μm Mixed-C,300×7.5 mm, 3) PLgel 3 μm Mixed-E, 300×7.5 mm, and 4) PLgel 3 μmMixed-E, 300×7.5 mm.

The calibration of the GPC columns was carried out using linearpoly(methyl methacrylate) from Polymer Standards Services. An RIdetector from Waters (Waters 410) was used as detector. The analysis wascarried out at a flow rate of 0.5 ml/min using DMAc at 70° C. as eluent.The GPC curves were evaluated using software from Millenium.

The following characteristic properties were determined by means of GPCanalysis both for the original NBR rubber (before degradation) and forthe degraded nitrile rubbers:

M_(w) [kg/mol]: weight average molar massM_(n) [kg/mol]: number average molar massPDI: width of the molar mass distribution (M_(w)/M_(n))

Example Series 1-3 Activity Comparison of the “Hoveyda Catalyst” Withthe “Grubbs II Catalyst” in the Presence of 2 Phr of 1-Hexene

In the example series 1 and 2, the activity of the “Hoveyda catalyst”was compared with that of the “Grubbs II catalyst” at two rutheniumcontents (23.8 ppm and 161.4 ppm). These activity comparisons werecarried out using 2.0 phr of 1-hexene.

In example series 3, the metathetic degradation was carried out usingthe “Hoveyda catalyst” at a ruthenium addition of 8.6 ppm and an amountof 1-hexene of 2.0 phr.

Example 1.1 According to the Invention

“Hoveyda Catalyst” Using 161 ppm of Ruthenium and 2.0 phr of 1-hexene

“Hoveyda catalyst” NBR (MW = 626.14 g/mol) 1-Hexene Temper- AmountAmount Amount Ru Amount Amount ature [g] [mg] [phr] [ppm] [g] [phr] [°C.] 40 40 0.1 161.4 0.8 2.0 23 Analy- tical “Hoveyda catalyst”/Reactiontime [min.] data 0 30 60 185 425 M_(w) 240 22 16 14 13 [kg/mol] M_(n)100 14 12 10 10 [kg/mol] PDI  2.4  1.57  1.33  1.40  1.30

The degraded nitrile rubbers obtained in Example 1.1 were gel-free.

Example 1.2 Comparison

“Grubbs II Catalyst” Using 161 ppm of Ruthenium and 2.0 phr of 1-hexene

NBR “Grubbs II catalyst” Temper- Amount Amount Amount Ru 1-Hexene ature[g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 54.2 0.14 161.4 0.8 2.0 23Analy- tical “Grubbs II catalyst”/Reaction time [min.] data 0 30 60 185425 M_(w) 240 86 65 35 28 [kg/mol] M_(n) 100 40 35 19 17 [kg/mol] PDI  2.4   2.13   1.87   1.88   1.68

Comparison of the decrease in the molecular weights M_(w) and M_(r) inExamples 1.1 and 1.2 shows that at an amount of ruthenium of 161 ppm theactivity of the “Hoveyda catalyst” is significantly higher than that ofthe “Grubbs II catalyst”. When using the “Hoveyda catalyst”, themetathesis reaction is complete after about 30 minutes, while when the“Grubbs II catalyst” is used the metathetic degradation is still notcomplete after a reaction time of 425 minutes. At in each case the samereaction times, the “Hoveyda catalyst” gives lower molar masses than the“Grubbs II catalyst”.

Example 2.1 According to the Invention

“Hoveyda Catalyst” Using 23.8 ppm of Ruthenium and 2.0 phr of 1-hexene

“Hoveyda catalyst” NBR (MW = 626.14 g/mol) 1-Hexene Temper- AmountAmount Amount Ru Amount Amount ature [g] [mg] [phr] [ppm] [g] [phr] [°C.] 40 5.9 0.014 23.8 0.8 2.0 23 Analy- tical “Hoveydacatalyst”/Reaction time [min] data 0 30 60 185 425 M_(w) 240 79 77 72 70[kg/mol] M_(n) 100 55 40 40 39 [kg/mol] PDI  2.4  1.44  1.90  1.82  1.77

The nitrile rubbers degraded using the “Hoveyda catalyst” in the trial2.1 were gel-free.

Example 2.2 Comparison

“Grubbs II Catalyst” Using 23.8 ppm of Ruthenium and 2.0 phr of 1-hexene

NBR “Grubbs II catalyst” Temper- Amount Amount Amount Ru 1-Hexene ature[g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 8 0.02 23.8 0.8 2.0 23 Analy-tical “Grubbs II catalyst”/Reaction time [min.] data 0 30 60 185 4251325 M_(w) 240 190 180 150 125 118 [kg/mol] M_(n) 100  67  63  59  62 51 [kg/mol] PDI  2.4  2.83  2.86  2.54  2.02  2.31

Comparison of the molecular weights M_(w) and M_(n) in the trials 2.1and 2.2 shows that at identical amounts of ruthenium (24 ppm) and of1-hexene (2 phr) the activity of the Hoveyda catalyst (V) issignificantly higher than that of the Grubbs II catalyst. At in eachcase identical reaction times, the Hoveyda catalyst (V) gave lower molarmasses than the Grubbs II catalyst.

Example 3 According to the Invention

“Hoveyda Catalyst” Using 8.6 ppm of Ruthenium and 2.0 phr of 1-hexene

NBR “Hoveyda catalyst” Temper- Amount Amount Amount Ru 1-Hexene ature[g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 2.12 0.0053 8.56 0.8 2.0 23Analy- tical “Hoveyda catalyst”/Reaction time [min] data 0 30 60 185 4251325 M_(w) 240 154 136 121 100 86 [kg/mol] M_(n) 100  69  60  54  48 45[kg/mol] PDI  2.4  2.23  2.27  2.24  2.08  1.91

A comparison of Example 3 (“Hoveyda catalyst”/8.6 ppm of ruthenium) with2.2 (“Grubbs II catalyst”/23.8 ppm of ruthenium) showed that the meanmolecular weights M_(w) and M_(n) at identical reaction times are lowerwhen using the “Hoveyda catalyst” than when using the “Grubbs IIcatalyst” despite a significantly lower amount of ruthenium.

The nitrile rubbers degraded using the “Hoveyda catalyst” in thisexample series 3 were also gel-free.

Example Series 4 and 5 Activity Comparison of the “Grela Catalyst” withthe “Grubbs II Catalyst” in the Presence Of 2 phr of 1-hexene

In the example series 4 and 5, the activity of the “Grela catalyst” wascompared with that of the “Grubbs II catalyst” at two ruthenium contents(23-24 ppm and 60 ppm). The activity comparisons were in each casecarried out using 2.0 phr of 1-hexene.

Example 4.1 According to the Invention

“Grela Catalyst” Using 22.9 ppm of Ruthenium and 2 phr of 1-hexene

“Grela catalyst” NBR (MW: 671.13 g/mol) 1-Hexene Temper- Amount AmountAmount Ru Amount Amount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 406.1 0.015 22.9 0.8 2.0 23 “Grela catalyst”/ Analytical Reaction time[min.] data 0 30 60 185 425 M_(w) 240 109 98 — 86 [kg/mol] M_(n) 100 6150 — 44 [kg/mol] PDI 2.4 1.79 1.96 — 1.95

Example 4.2 Comparison

“Grubbs II Catalyst” Using 23.8 ppm of Ruthenium and 2 phr of 1-hexene

NBR “Grubbs II catalyst” 1-Hexene Temper- Amount Amount Amount Ru AmountAmount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 8.0 0.02 23.8 0.82.0 23 Analytical “Grubbs II catalyst”/Reaction time [min.] data 0 30 60185 425 M_(w) 240 190 180 150 125 [kg/mol] M_(n) 100 67 63 59 62[kg/mol] PDI 2.4 2.83 2.86 2.54 2.02

Comparison of the mean molecular weights M_(w) and M_(n) in the trials4.1 and 4.2 shows that at identical amounts of ruthenium (22.9/23.8 ppm)and of 1-hexene (2 phr) the activity of the “Grela catalyst” issignificantly higher than that of the “Grubbs II catalyst”. At in eachcase identical reaction times, the “Grela catalyst” gave lower molecularweights than the “Grubbs II catalyst”.

The nitrile rubbers degraded using the “Grela catalyst” in the trial 4.1were gel-free.

Example 5.1 According to the Invention

“Grela Catalyst” Using 59.5 ppm of Ruthenium and 2.0 phr of 1-hexene

“Grela catalyst” NBR (MW: 671.13 g/mol) 1-Hexene Temper- Amount AmountAmount Ru Amount Amount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 4015.8 0.04 59.5 0.8 2.0 23 Analy- tical “Grela catalyst”/Reaction time[min.] data 0 30 60 185 425 M_(w) 240 37 35 31 31 [kg/mol] M_(n) 100 2322 20 20 [kg/mol] PDI  2.4  1.61  1.59  1.50  1.55

Example 5.2 According to the Invention

“Grubbs II Catalyst” Using 59.6 ppm of Ruthenium and 2.0 phr of 1-hexene

NBR “Grubbs II catalyst” 1-Hexene Temper- Amount Amount Amount Ru AmountAmount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 20.0 0.05 59.6 0.82.0 23 Analytical “Grubbs II catalyst”/Reaction time [min] data 0 30 60185 425 M_(w) 240 185 165 77 60 [kg/mol] M_(n) 100  84  78 38 35[kg/mol] PDI  2.4  2.13  2.11  2.03  1.71

Comparison of the mean molecular weights M_(w) and M_(n) in the trials5.1 and 5.2 shows that at identical amounts of ruthenium (60 ppm) and of1-hexene (2 phr) the activity of the “Grela catalyst” is significantlyhigher than that of the “Grubbs II catalyst”. When using the “Grelacatalyst”, final values for M_(w) and M_(n) were achieved after about185 minutes, while when the “Grubbs II catalyst” was used the metatheticdegradation was not yet complete after a reaction time of 425 minutes.At in each case identical reaction times, the “Grela catalyst” gavelower molecular weights than the “Grubbs II catalyst”.

The nitrile rubbers degraded using the “Grela catalyst” in the trial 5.1were gel-free.

Example 6 Activity Comparison of the “Hoveyda Catalyst” with the “GrubbsII Catalyst” without Addition of 1-hexene

In the trials 6.1 and 6.2, the activity of the “Hoveyda catalyst” wascompared with that of the “Grubbs II catalyst” at a ruthenium content of60 ppm. The activity comparison was carried out without the use of1-hexene.

Example 6.1 According to the Invention

“Hoveyda Catalyst” Using 59.3 ppm of Ruthenium without Addition of1-hexene

NBR “Hoveyda catalyst” 1-Hexene Temper- Amount Amount Amount Ru AmountAmount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 14.7 0.0368 59.3 00 23 Analytical “Hoveyda catalyst”/Reaction time [min.] data 0 30 60 185425 M_(w) 240 149 143 132 108 [kg/mol] M_(n) 100  66  63  62  54[kg/mol] PDI  2.4  2.26  2.27  2.13  2.00

Example 6.2 Comparison

“Grubbs II Catalyst” Using 59.6 ppm of Ruthenium without Addition of1-hexene

NBR “Grubbs II catalyst” 1-Hexene Temper- Amount Amount Amount Ru AmountAmount ature [g] [mg] [phr] [ppm] [g] [phr] [° C.] 40 20.0 0.05 59.6 0 023 Analy- tical “Grubbs II catalyst”/Reaction time [min.] data 0 30 60185 425 M_(w) 240 206 — 173 158 [kg/mol] M_(n) 100  92 —  74  75[kg/mol] PDI  2.4  2.24 —  2.34  2.11

Comparison of the mean molecular weights M_(w) and M_(n) in Examples 6.1and 6.2 shows that at a comparable amount of ruthenium (about 60 ppm)without addition of 1-hexene the activity of the “Hoveyda catalyst” ishigher than that of the “Grubbs II catalyst”. When using the “Hoveydacatalyst”, the mean molecular weights M_(w) and M_(n) are lower thanwhen using the “Grubbs II catalyst” at identical reaction times.

The degraded nitrile rubbers obtained using the “Hoveyda catalyst”without use of 1-hexene in Example 6.1 were gel-free.

What is claimed is:
 1. A process for degrading a nitrile rubbercomprising subjecting the nitrile rubber to a metathesis reaction in thepresence of the general formula (I),

where M is ruthenium or osmium, Y is oxygen (O), sulphur (S), an N—R¹radical or a P—R¹ radical, X¹ and X² are identical or different ligands,R¹ is an alkyl, cycloalkyl, alkenyl, alkynyl, aryl, alkoxy, alkenyloxy,alkynyloxy, aryloxy, alkoxycarbonyl, alkylamino, alkylthio, arylthio,alkylsulphonyl or alkylsulphinyl radical, each of which may optionallybe substituted by one or more alkyl, halogen, alkoxy, aryl or heteroarylradicals, R², R³, R⁴, R⁵ are identical or different and are eachhydrogen, organic or inorganic radicals, R⁶ is hydrogen or an alkyl,alkenyl, alkynyl or aryl radical and L is a ligand.
 2. The processaccording to claim 1, wherein L in the general formula (I) is a P(R⁷)₃radical, where the radicals R⁷ are each, independently of one another,C₁-C₆-alkyl, C₃-C₈ cycloalkyl or aryl or a substituted or unsubstitutedimidazolidine radical (“Im”).
 3. The process according to claim 2,wherein the imidazolidine radical (Im) has a structure of the generalformula (IIa) or (IIb),

where R⁸, R⁹, R¹⁰, R¹¹ are identical or different and are each hydrogen,straight-chain or branched C₁-C₃₀-alkyl, C₃-C₂₀-cycloalkyl,C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl, C₁-C₂₀-carboxylate,C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₀-aryloxy,C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylthio, C₆-C₂₀-arylthio,C₁-C₂₀-alkylsulphonyl, C₁-C₂₀-alkylsulphonate, C₆-C₂₀-arylsulphonate orC₁-C₂₀-alkylsulphinyl.
 4. The process according to claim 3, wherein R¹⁰and R¹¹ in the imidazolidine radical (Im) are identical or different andare each straight-chain or branched C₁-C₁₀-alkyl, C₃-C₁₀-cycloalkyl,C₆-C₂₄-aryl, C₁-C₁₀-alkylsulphonate, or C₆-C₁₀-arylsulphonate.
 5. Theprocess according to claim 3, wherein the imidazolidine radical (Im) hasthe structure (IIIa), (IIIb), (IIIc), (IIId), (IIIe) or (IIIf),

where Mes is in each case a 2,4,6-trimethylphenyl radical.
 6. Theprocess according to claim 1 or 2, wherein X¹ and X² in the generalformula (I) are identical or different and are each hydrogen, halogen,pseudohalogen, straight-chain or branched C₁-C₃₀-alkyl, C₆-C₂₄-aryl,C₁-C₂₀-alkoxy, C₆-C₂₄-aryloxy, C₃-C₂₀-alkyldiketonate,C₆-C₂₄-aryldiketonate, C₁-C₂₀-carboxylate, C₁-C₂₀-alkylsulphonate,C₆-C₂₄-arylsulphonate, C₁-C₂₀-alkylthiol, C₆-C₂₄-arylthiol,C₁-C₂₀-alkylsulphonyl or C₁-C₂₀-alkylsulphinyl.
 7. The process accordingto claim 6, wherein X¹ and X² in the general formula (I) are identicalor different and are each halogen, benzoate, C₁-C₅-carboxylate,C₁-C₅-alkyl, phenoxy, C₁-C₅-alkoxy, C₁-C₅-alkylthiol, C₆-C₂₄-arylthiol,C₆-C₂₄-aryl or C₁-C₅-alkyl sulphonate.
 8. The process according to claim6, wherein X¹ and X² in the general formula (I) are identical and areeach halogen, CF₃COO, CH₃COO, CFH₂COO, (CH₃)₃CO, (CF₃)₂(CH₃)CO,(CF₃)(CH₃)₂CO, PhO (phenoxy), MeO (methoxy), EtO (ethoxy), tosylate(p-CH₃—C₆H₄—SO₃), mesylate (2,4,6-trimethylphenyl) or CF₃SO₃(trifluoromethanesulphonate).
 9. The process according to claim 1 or 2,wherein the radical R¹ in the general formula (I) is a C₁-C₃₀-alkyl,C₃-C₂₀-cycloalkyl, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl,C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₄-aryloxy,C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylamino, C₆-C₂₄-arylthio,C₁-C₂₀-alkylsulphonyl or C₁-C₂₀-alkylsulphinyl radical, each of whichmay optionally be substituted by one or more alkyl, halogen, alkoxy,aryl or heteroaryl radicals.
 10. The process according to claim 1 or 2,wherein the radicals R², R³, R⁴ and R⁵ in the general formula (I) areidentical or different and are each hydrogen, halogen, nitro, CF₃ or aC₁-C₃₀-alkyl, C₃-C₃₀-cycloalkyl, C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl,C₆-C₂₄-aryl, C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy,C₆-C₂₄-aryloxy, C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylamino,C₁-C₂₀-alkylthio, C₆-C₂₄-arylthio, C₁-C₂₀-alkylsulphonyl orC₁-C₂₀-alkylsulphinyl radical, each of which may optionally besubstituted by one or more C₁-C₃₀-alkyl, C₁-C₂₀-alkoxy, halogen,C₆-C₂₄-aryl or heteroaryl radicals.
 11. The process according to claim 1or 2, wherein the radical R⁶ is hydrogen, a C₁-C₃₀-alkyl radical, aC₂-C₂₀-alkenyl radical, a C₂-C₂₀-alkynyl radical or a C₆-C₂₄-arylradical.
 12. The process according to claim 1, wherein a catalyst of thegeneral formula (IV)

where L, X¹, X², R¹, R², R³, R⁴ and R⁵ have the meanings given for thegeneral formula (I), is used.
 13. The process according to claim 12,wherein, in the general formula (IV), M is ruthenium, X¹ and X² are bothchlorine, R¹ is an isopropyl radical, R², R³, R⁴, R³ are all hydrogenand L is a substituted or unsubstituted imidazolidine radical of theformula (IIa) or (IIb),

where R⁸, R⁹, R¹⁰, R¹¹ are identical or different and are each hydrogen,straight-chain or branched C₁-C₃₀-alkyl, C₃-C₂₀-cycloalkyl,C₂-C₂₀-alkenyl, C₂-C₂₀-alkynyl, C₆-C₂₄-aryl, C₁-C₂₀-carboxylate,C₁-C₂₀-alkoxy, C₂-C₂₀-alkenyloxy, C₂-C₂₀-alkynyloxy, C₆-C₂₄-aryloxy,C₂-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylthio, C₆-C₂₄-arylthio,C₁-C₂₀-alkylsulphonyl, C₁-C₂₀-alkylsulphonate, C₆-C₂₄-arylsulphonate orC₁-C₂₀-alkylsulphinyl.
 14. The process according to claim 1, wherein acatalyst of the formula (V), (VI), (VII), (VIII), (IX), (X), (XI), (XII)or (IXX),

where Mes is in each case a 2,4,6-trimethylphenyl radical, is used. 15.The process according to claim 1, wherein catalysts of the generalformula (XIII),

where M, L, X¹, X², R¹ and R⁶ have the meanings given for the generalformula (I), the radicals R¹² are identical or different and have themeanings given for the radicals R², R³, R⁴ and R⁵ in the general formula(I), with the exception of hydrogen, and n is 0, 1, 2 or 3, are used.16. The process according to claim 1, wherein a catalyst of the formula(XIV) or (XV)

is used.
 17. The process according to claim 1, wherein a catalyst of thegeneral formula (XVI),

where D¹, D², D³ and D⁴ each have a structure of the general formula(XVII) which is bound via the methylene group to the silicon of theformula (XVI),

where M, L, X¹, X², R¹, R², R³, R⁵ and R⁶ have the meanings given forthe general formula (I), is used.
 18. The process according to claim 1,wherein a catalyst of the general formula (XVIII),

where the symbol

represents a support, is used.
 19. The process according to claim 1,wherein the amount of catalyst used is from 5 to 1000 ppm of noble metalbased on the nitrile rubber used.
 20. The process according to claim 1,wherein the metathesis is carried out without a coolefin or in thepresence of a coolefin.
 21. The process according to claim 1, whereincopolymers or terpolymers which comprise repeating units of at least oneconjugated diene, at least one α,β-unsaturated nitrile and, one or morefurther copolymerizable monomers are used as nitrile rubbers.
 22. Theprocess according to claim 1, wherein the nitrile rubbers used have aMooney viscosity (ML 1+4 at 100° C.) in the range from 30 to
 70. 23. Theprocess according to claim 1 wherein the degraded nitrile rubber issubsequently subjected to a hydrogenation.
 24. The process according toclaim 3, wherein R¹⁰ and R¹¹ in the imidazolidine radical (Im) areidentical or different and are each straight-chain or branchedC₁-C₁₀-alkyl selected from the group consisting of i-propyl orneopentyl.
 25. The process according to claim 3, wherein R¹⁰ and R¹¹ inthe imidazolidine radical (Im) are identical or different and are eachstraight-chain or branched C₃-C₁₀-cycloalkyl selected from adamantyl.26. The process according to claim 3, wherein R¹⁰ and R¹¹ in theimidazolidine radical (Im) are identical or different and are eachstraight-chain or branched C₆-C₂₄-aryl selected from phenyl.
 27. Theprocess according to claim 3, wherein R¹⁰ and R¹¹ in the imidazolidineradical (Im) are identical or different and are each straight-chain orbranched C₁-C₁₀-alkylsulphonate selected from methanesulphonate.
 28. Theprocess according to claim 3, wherein R¹⁰ and R¹¹ in the imidazolidineradical (Im) are identical or different and are each straight-chain orbranched C₆-C₁₀-arylsulphonate selected from p-toluenesulphonate. 29.The process according to claim 6, wherein X¹ and X² in the generalformula (I) are identical or different and are each halogen selectedfrom the group consisting of fluorine, chlorine or bromine.
 30. Theprocess according to claim 1 or 2, wherein the radicals R², R³, R⁴ andR⁵ in the general formula (I) are identical or different and are halogenselected from the group containing chlorine or bromine.
 31. The processaccording to claim 1, wherein a catalyst of the general formula (XVIII),

where the symbol

represents a support consisting of a poly(styrene-divinylbenzene)copolymer (PS-DVB).
 32. The process according to claim 19, wherein theamount of catalyst used is from 5 to 500 ppm based on the nitrile rubberused.
 33. The process according to claim 32, wherein the amount ofcatalyst used is in the range from 5 to 250 ppm based on the nitrilerubber used.
 34. The process according to claim 1, wherein themetathesis is carried out without a coolefin or in the presence of acoolefin which is a straight-chain or branched C₂-C₁₆-olefin.
 35. Theprocess according to claim 34, wherein the straight-chain or branchedC₂-C₁₆-olefin, selected from the group consisting of ethylene,propylene, isobutene, styrene, 1-hexene or 1-octene.
 36. The processaccording to claim 22, wherein the nitrile rubbers used have a Mooneyviscosity (ML 1+4 at 100° C.) in the range from 30 to
 50. 37. Theprocess according to claim 23 wherein the degraded nitrile rubber issubsequently subjected to a hydrogenation in situ.